In known fashion alkaline silicate glasses, in particular borosilicate glasses, are used for gas discharge lamps, as, for example, CCFL-(cold-cathode fluorescent lamp) or even EEFL-(external electrode fluorescent lamp)-gas discharge lamps. The main components of such glasses are SiO2, B2O3, as well as alkali and alkaline earth oxides. Because of the relatively high content of, for example, between 10 and 20 percent by weight of B2O3 in the glass matrix, these glasses have a relatively poor chemical consistency toward acids and bases, as well as low hydrolytic consistency.
It is also known to subject the glass tubes to a washing process for the purpose of cleaning them before the actual production process of the lamps. On the one hand, this washing process serves the purpose to remove contamination which settled, for example, during the period of transporting on the outer and inner surface. On the other hand, it serves the purpose of removing possibly existing glass splinters. The washing process is performed with water using, if required, cleaning supplements.
This usually performed washing process has a number of disadvantages: Because of the poor chemical consistency of the glasses, it is possible that, during the washing process, the glass surface has a reaction to the washing solution, for example, a lixiviating reaction. This can result in dissolution of the glass, i.e., the glass is etched or completely dissolved, and components of the glass are dissolved. Although under normal circumstances, such as at room temperature and having a neutral pH value, the glass is only partially etched, such circumstances alter the form as well as the properties of the glass surface. Furthermore, an ion exchange can take place at the glass surface during which, for example, alkali ions from the glass can be dissolved and exchanged with protons (H+). It is also possible that B2O3 is dissolved from the glass matrix.
The negative effects of these reactions of the washing solution on the glass appear especially during further processing of the glasses: During subsequent heat treatments, for example, a subsequent tempering step, for example, the so-called baking step (at maximum temperatures in the range of, for example, between app. 500° C. and app. 750° C., typically between 600° C. and 700° C.), during which the fluorescent layer is burned on the inner surface of the tube, results in shrinkage or compaction of the tube. In the context of the invention at hand, the term compaction refers to an irreversible local densification corresponding to a contraction of the material on molecular level. By dissolving components, such as alkali ions or B2O3, from the glass matrix, the network becomes coarsely meshed, as it were, so that the network tightens during the process of heating the tube.
This can result in form deformation, especially in a reduction of the tube length. This reduction of the tube can range between 0.5% and 10% or more. In worst case scenario it means that because of the tube reduction the tubes are useless for the intended purpose and have to be eliminated as waste.
Besides with glass, such adverse effects are expected to occur also with other materials.
Surprisingly, the inventors discovered that it is possible to considerably reduce or completely prevent this unwanted shrinkage by performing the washing process under precisely defined conditions, especially with a pH value ranging from neutral to slightly alkaline, i.e., between 7 and 9, preferably ranging between 7 and 8. A subsequent processor can therefore use directly as a finished product a washing solution with an adjusted pH value. It is also possible to use a washing solution in which the pH value in the range mentioned is subsequently adjusted, for example, by adding a pH buffer system in order to eliminate the adverse effects described. However, washing has the great disadvantage that the producer of the substrate, such as a glass manufacturer, has no influence on the proper and optimum adjustment of the pH value of the washing solution and has no possibility of controlling this production step with regard to quality control. However, improper performance of the washing process on the part of the subsequent processor can result in material defects for which the producer [of the substrate] can be blamed, accusing him of having provided defective material.
Accordingly, the invention at hand is based on the objective of preventing the disadvantages of prior art described above and to provide the possibility of minimizing the reaction of a washing substance in a washing process involving a material surface, or to control such reaction in a defined manner, for example, to process in a controlled way the shrinkage in the form of material reduction with subsequent temperature treatment in order to achieve on an average the best possible uniform change over a large number of batches. In this connection the control of the washing process should be performed in such a way that even a subsequent user would be able to perform the washing process in a controlled manner. Furthermore, the washing process should fulfill its original purpose of cleaning in a satisfying way the material surface.